1. Field of the Invention
The present invention relates to a multiple antenna system, and more particular, to an apparatus and method for determining whether to feedback each channel information of mobile station in a multiple antenna system in a multi-user environment.
2. Description of the Related Art
Recently, with the rapid growth of a wireless telecommunication market, there is a demand for a variety of multimedia services in a wireless environment. To meet this demand, transmission data becomes large in size, and data transmission speed increases. Research on a multiple antenna system (e.g., Multiple Input Multiple Output (MIMO)) has been conducted so a limited radio resource can be effectively used in a wireless telecommunication system.
In a multiple antenna system, by utilizing mutually independent channels for respective antennas, data transmission speed can be maximized, and channel fading can be avoided. Compared with a system using a single antenna, a multiple antenna system can increase channel transmission capacity in proportion to the number of antennas without having to additionally allocate a frequency or transmission power. As a result, research thereon has been actively conducted recently.
Furthermore, in a multiple antenna system, system reliability of the multiple antenna system can be improved according to a diversity gain, and a transmission rate can be improved according to a multiplexing gain. A multiple antenna system may operate in a multi-user environment in order to support multi-users.
In a multiple antenna system operating in a multi-user environment, a channel state of each mobile station (or user) is checked for, and data transmission is granted to a mobile station having the highest channel gain. In this manner, a multi-user diversity gain can be obtained. That is, multi-user diversity gain can be obtained by performing a scheduling process whereby data transmission is granted in consideration of the multi-user channel gain. The multi-user diversity gain has a diversity order higher than a diversity gain of an antenna which is taken into account in a multiple antenna system in a single user environment. Thus, it is possible to increase a transmission rate of the multiple antenna system.
Examples of a scheduling method of a multiple antenna system in a multi-user environment include a Best User selection (BU) scheduling method, a Best Substream selection (BS) scheduling method, a Proportional Fair (PF) scheduling method, and an eigen-based scheduling method. In a BU scheduling method, all streams are allocated to one mobile station of which a channel has the highest average channel gain of transmission antennas. In a BS scheduling method, a mobile station of which a channel has the highest channel gain is selected for each antenna to allocate a stream. In a PF scheduling method, an antenna is fairly allocated to each mobile station. In an eigen-based scheduling method, a singular vector corresponding to an antenna which has the highest channel gain and is used by each mobile station is fed back so a mobile station having the lowest correlation is selected.
The aforementioned scheduling methods are used to select a mobile station in a multiple antenna system in a multi-user environment, but there is a problem in that performance is degraded in a correlation channel environment. That is, correlation exists among antennas in a practical environment where a multiple antenna system is used. Therefore, a rank of the multiple antenna system decreases in a parallel channel environment. As a result, diversity gain that can be obtained by using the scheduling methods also decreases in a multiple antenna system. In an eigen-value based scheduling method, scheduling is performed by selecting a mobile station having the lowest correlation in order to avoid performance degradation caused by the correlation between antennas. However, the diversity gain cannot be fully achieved in a multi-user environment.
Meanwhile, a receiving end of the multiple antenna system feeds back channel information to a transmitting end so that scheduling can be performed at the transmitting end. However, this feedback process may cause a waste of resource in a multiple antenna system. Furthermore, channel information is fed back as many times as an increased number of transmission antennas or reception antennas in a multiple antenna system. For this reason, the receiving end of a multiple antenna system feeds back only a part of channel information to the transmitting end. Scheduling cannot be accurately performed in the transmitting end of a multiple antenna system, resulting in performance degradation. Accordingly, there is a need for a method in which a high transmission rate can be achieved in a multiple antenna system by feeding back only a necessary part of the channel information.